I shouldn't have any excuses either: I'm doing them for challenges so I should provide only quality icons (most of the time, there's only 1 to 3 I'm really satisfied with on a batch). Anyway, depending on the kind of icon you're working on, it'll be easy to hide the small mistakes/laziness. Me neither, to be honest, I've only used brushes for a couple months. Before that, it was mostly, coloring, a few textures and that was all XD But you really can do some wonders only with that, even more complexe compositions.

Nope, you don't have to merge anything before you save. When it export the icons to png, it merges the layers automatically. Doesn't improve quality. Personally, even though I don't keep the psd/xcf, I never merge mine before saving.